This invention relates generally to abradable fluid seals such as, for example, those used to provide a relatively fluid tight seal between the tips of turbine blades and the surrounding casing in an aircraft gas turbine engine.
More specifically, this invention relates to the honeycomb steel band which is laid along and brazed to a strong annular steel base to form part of a turbine shroud and whose hexagonal cell structure is filled with an abradable material such as a sintered nickel-alloy to provide thermal insulation for the annular steel base. Such conventional annular seals are affixed to the turbine casing so that the abradable material in the honeycomb is very closely spaced from the tips of the rotating turbine blades. The abradable material in the honeycomb cells can readily withstand the high temperatures encountered in the turbine stages of the engine and protects the annular steel backing from high temperatures as well as providing a smooth gas flow surface. However, under extreme operating conditions, temperatures, etc., the turbine blades may expand sufficiently to actually rub the surface of the abradable alloy overfilling the honeycomb material, whereupon the abradable sealant material will yield so that the turbine blades will not be damaged. An example of such a conventional honeycomb steel band is disclosed in U.S. Pat. No. 2,963,307 issued to M. Bobo on Dec. 6, 1960. Variations of this basic honeycomb structure are disclosed in U.S. Pat. No. 3,867,061 issued to Seymour Moskowitz, on Feb. 18, 1975 and in U.S. Pat. No. 4,063,742 previously issued to me on Dec. 20, 1977.
One difficulty that has been encountered using the abradable nickel-alloy filler in conventional honeycomb is that, over a relatively long period of time, the abradable material filling a given cell of the honeycomb can become loosened and fall out due to thermal cycling, rubbing of the abradable filler against the turbine blades, engine vibrations, or can otherwise be blown out by action of the hot gas flowing across the seal. It is thus highly desirable to provide an improved honeycomb structure with an increased ability to hold and confine the abradable alloy filler to thus minimize the quantities of such filler that will be chipped out of the honeycomb when the filler surface is rubbed by the tips of the turbine blades.
My present invention substantially overcomes this and other difficulties which have long been encountered using the conventional honeycomb in abradable seals of the gas turbine prior art.